1. Field of the Invention
The present invention related generally to the fabrication of semiconductor pixel sensor arrays, and more particularly, to a novel semiconductor pixel structure and novel process therefor for increasing the sensitivity of the pixel sensors by optimizing the formed microlens structure.
2. Discussion of the Prior Art
CMOS image sensors are beginning to replace conventional CCD sensors for applications requiring image pick-up such as digital cameras, cellular phones, PDA (personal digital assistant), personal computers, and the like. Advantageously, CMOS image sensors are fabricated by applying present CMOS fabricating process for semiconductor devices such as photodiodes or the like, at low costs. Furthermore, CMOS image sensors can be operated by a single power supply so that the power consumption for that can be restrained lower than that of CCD sensors, and further, CMOS logic circuits and like logic processing devices are easily integrated in the sensor chip and therefore the CMOS image sensors can be miniaturized.
The patent literature is replete with references describing image sensor arrays having a microlens structure and aspects of their manufacture. United States Patent Publication Nos. 2002/0034014, 2004/0099633, 2004/0146807, 2004/0147059 and 2004/0156112 describe state of the art microlens structures and methods of manufacture for image arrays. Basically, the typical method for fabricating a microlens structure comprises: first a layer of photoresist is applied, e.g., by spin coating or like application process (e.g., dip coating, chemical vapor deposition, brushing, evaporation and other like deposition techniques), atop a wafer surface. For instance, as shown in FIG. 1(a), the wafer surface may comprise a dielectric planarization layer formed over a substrate 40 that includes an array of color filter structures each associated with an active light sensitive device (e.g., photodiode) of a pixel. It is understood that a positive or negative photoresist may be applied with attendant photolithographic processing steps applied; however, for purposes of discussion, it is assumed a negative photoresist is applied. After a soft bake process, a photoresist mask, such as chrome on glass, is applied having a patterned grid comprising a two-dimensional array of translucent squares or rectangle openings corresponding to the pixel microlens structure to be printed. After aligning the mask to the correct location, the mask and wafer are exposed to a controlled dose of UV light to transfer the mask image. In this example, polymer resist in each of the exposed square (or rectangular) regions are crosslinked so that these regions do not dissolve in a subsequent application of a developer chemical. Then, after a post-exposure bake process, a developer step is performed (the kinds of developers that can be employed are well known to those skilled in the art and are dependent on whether a positive or negative photoresist is employed) to remove the soluble areas of the photoresist leaving a visible array pattern of square (or rectangular) shaped islands 42 on the wafer surface separated by thin gaps. Then, as shown in FIG. 1(b), after a further blanket expose step, the photoresist pattern is subject to a heating and reflow process to convert the raised photoresist islands into semi-spherical convex lenses 45 of circular plan shape linearly aligned in correspondence with the color filter and active photoelectric converting device.
It is the case however, that the microlens structures formed in this manner exhibit light loss between the lenses. For example, FIG. 2 illustrates a cross-sectional view of a single image sensor (pixel cell) 50 including a formed microlens 75 on top of the active pixel element, e.g., a light sensitive photodiode 78 formed in the Silicon or Silicon containing substrate 64. As known, the microlens 75 are used in the pixel image sensors to focus incident light 80 into the active area in the pixel. Without the microlens, much of the light is not collected that would strike the cell and, even worse, some light would get reflected off of the interlevel metallization, e.g., Cu metal wires (not shown), formed in interlevel dielectric material layer 74 and strike adjacent cells which blur the image. As the technology scales to smaller pixels it becomes increasingly critical to collect as much light as possible by minimizing the space between the pixels. For instance, FIG. 2 depicts a raytrace model depicting normal incidence light 80 on a 2.75 um radius lens 75 on a 4.5 μm pixel cell. In this state of the art imaging sensor device 50, each adjacent microlens of the pixel array is separated by a gap 79 which acts to decrease the amount of light focused to the active photodiode element 78 of the pixel, thus, compromising the sensitivity of the imager pixel array.
Even if adjacent cells can be formed to touch, adjacent pixel cells may exhibit an ideal radius of curvature in horizontal cross-section, for example, as shown in FIG. 3 where the cross section 25 is taken along line A-A of a formed lens 50 exhibiting a radius of curvature at the horizontal edges. However, the cross section 30 taken along line B-B and angled at 45 degrees is not matched to the cross-section because the resultant lens 50 is square (driven by the pixel microlens structure being a square). The radius of curvature of the microlens structure particularly determines the focal length of the microlens, so as long as the vertical dimensions do not shrink—which is often the case—the microlens radius of curvature must remain the same as the cell shrinks. To achieve the same radius of curvature while the cell shrinks, the microlens thickness must be reduced to a point where it is very difficult to make. For example, a 2.2 μm cell would require a lens thickness of 360 nm and to achieve this dimension, a spin coating over topography would have to be below 300 nm.
It is further the case that the smaller the pixel size, the greater the percentage of light is wasted between the lenses.
It would be highly desirable to provide a pixel sensor and method of manufacture wherein the sensor includes a microlens structure having substantially no space between the microlens structure of adjacent pixels to thus maximize light collection, and further that are fabricated in a manner such that adjacent microlens are fully formed having uniform radius of curvature at the cross-section and angled cuts so as to maximize light being focused into the active pixel element.